JP-A-2004-94050 discloses a monitoring camera having a visible light camera to be used in the daytime and an infrared camera to be used at nighttime. According to JP-A-2004-94050, the two cameras share a single optical system, and visible light and infrared light which have passed through the optical system are divided by a beam splitting mirror toward the two cameras, respectively, to allow photographing with the two cameras. It is therefore possible to perform photographing day and night.
However, in the lens device disclosed in JP-A-2004-94050, since the quantity of visible light which has passed through the optical section is halved by the beam splitting mirror, the quantity of light incident on the visible light camera becomes small, and a problem has therefore arisen in that a clear image can not be obtained with the visible light camera.